1. Field of the Invention
The present invention relates to demultiplexer device for wavelength-division multiplexed optical fiber communication, and particularly to demultiplexer device for wavelength-division multiplexed optical fiber communication which is advantageous for efficiently performing the dispersion compensation and loss compensation of an optical signal.
2. Description of the Related Art
In the conventional wavelength-division multiplexed optical fiber communication system, the following method was taken to demultiplex an optical wavelength-division multiplexed signal. FIG. 3 is a diagram showing the conventional demultiplexer device. An wavelength-division multiplexed optical signal transmitted through an optical fiber 1 is dispersion compensated by an optical fiber 2 for dispersion equalization (hereinafter simply referred to as "equalizing fiber"), and a loss during a transmission is compensated by amplification in a optical amplifier 3 and inputted to a demultiplexer device 4. In this example, the number of multiplexings is assumed to be "4".
The demultiplexer device 4 comprises an optical fiber coupler (hereinafter simply referred to as "coupler") 5, equalizing fibers 6 to 11, optical amplifiers 12 to 14, and optical fibers 15 to 18. The coupler 5 is a 1.times.4 coupler for branching the wavelength-division multiplexed optical signal into four directions. The wavelength-division multiplexed optical signal inputted to the demultiplexer device 4 is branched into four directions by the 1.times.4 coupler 5, and the respective wavelength-division multiplexed optical signals are inputted to the optical filters 15 to 18 and branched into optical signals S1 to S4 (wavelengths .lambda.4 to .lambda.1 correspond to the signals S1 to S4 respectively) according to the characteristics of the respective optical filters. The wavelengths of the respective optical signals are in a relationship of .lambda.1&lt;.lambda.2&lt;.lambda.3&lt;.lambda.4, and the wavelength spacings are equal. Since generally an optical signal of a shorter wavelength is more susceptible to dispersion because of the zero dispersion wavelength of the transmission path, it requires a longer equalizing fiber.
Accordingly, as shown, equalizing fibers 6 to 11 are disposed in the channels corresponding to the optical signals S2 to S3 which have shorter wavelength. In particularly, channels for the optical signals with shorter wavelength have many equalizing fibers. No additional equalizing fiber is disposed for optical signal S1 with long wavelength, because the optical signal S1 is fully compensated for dispersion by the equalizing fiber 2. Since the equalizing fiber attenuates an optical signal, the optical amplifiers 12 to 14 are respectively disposed in the downstream of the equalizing fibers 6, 8 and 11. The optical fibers 15 to 18 may be provided immediately after the coupler 5.
The above conventional demultiplexer device has the following problems. As seen from FIG. 3, in the conventional demultiplexer device, it is needed to dispose equalizing fibers for the respective channels correspondingly to all the optical signals S2 to S4 other than the optical signal S1 with the longest wavelength. Further, it is required to dispose an optical amplifier for each channel as the dispersion equalizing optical filters are disposed. As a result, the number of parts increases and hence the improvement of the system efficiency and reliability cannot be expected, and there is also a problem with economy. Specifically, since wavelengths more than those exemplified in FIG. 3, for instance, 8 to 16 or more, may be multiplexed, the problems are more significant in a practical use.